The present application claims benefit of priority under 35 U.S.C. xc2xa7119 to the following Japanese patent Applications Nos. 2000-364387 (filed on Nov. 30, 2000) and 2001-360940 (filed on Nov. 27, 2001), each of which are incorporated herein by reference.
(a) Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating the same, and more particularly to a so-called buried semiconductor laser device having a higher laser emission efficiency and a higher reproducibility of a current-optical output characteristic.
(b) Description of the Related Art
A semiconductor laser device having a lower threshold current density and a higher laser emission efficiency is desirable. A strained quantum well semiconductor laser device having a hetero-structure and a pair of current blocking layers is attracting public attention because of the excellent characteristics thereof. The semiconductor laser having a pair of current blocking layers in abutment to the semiconductor laser structure is generally called a buried semiconductor laser.
A conventional strained quantum well semiconductor laser device shown in JP-A-8(1996)-288589 will be described referring to FIG. 1A.
As shown in idealized form in FIG. 1A, a conventional strained quantum-well semiconductor laser device 20 includes a layer structure having an n-type InGaP bottom cladding layer 2, an active layer 3, and a p-type InGaP top cladding layer 4, sequentially and epitaxially grown on an n-type GaAs substrate 1 by using a metal organic chemical vapor deposition (MOCVD) method.
The active layer 3 is a five-layered structure including an InGaAsP layer 5, a GaAs layer 6, an InGaAs layer 7, a GaAs layer 8 and an InGaAsP layer 9.
The top cladding layer 4, the active layer 3 and the top part of the bottom cladding layer 2 are configured to have a mesa structure 11. Each of the side surfaces 12 of the mesa structure 11 and the adjacent surfaces of the bottom cladding layer 2 are covered with a p-type InGaP current blocking layer 14 and an n-type InGaP current blocking layer 15, which are sequentially deposited.
A second p-type InGaP top cladding layer 16 and a p-type contact layer 17 are sequentially deposited on the n-type InGaP current blocking layer 15, the p-type InGaP current blocking layer 14 and the top cladding layer 4 of the mesa structure 11.
A p-side metal electrode layer 18 and an n-side metal electrode layer 19 are deposited on the top surface of the p-type contact layer 17 and the bottom surface of the substrate 1, respectively.
The above publication points out a problem when the p-type current blocking layer 14 and the n-type current blocking layer 15 are grown by using an etching mask. Referring to FIG. 1B, structural defects such as hollows and grooves 40 are formed on the n-type current blocking layer 15 along the bottom surface of the etching mask due to the difference between the growth rates.
When the hollows 40 on the n-type current blocking layer 15 are large, crystal dislocations are liable to occur along the lines 41 shown in FIG. 1B. The propagation of a crystal dislocation from a point within layer 15 to a point within the p-type contact layer 17 increases the threshold current of the fabricated laser device, which lowers the laser emission efficiency.
The above publication describes the growth conditions of the p-type and n-type current blocking layers 14, 15 such that the substrate temperature is between 750xc2x0 C., and 800xc2x0 C. and a mixing ratio (concentration ratio) of a group V element gas with respect to a group III element gas is between 400:1 and 800:1 inclusive (V:III), thereby suppressing the occurrence of the structural defects (e.g., hollows) to decrease the probability and magnitude of the crystal dislocations. (As used later herein, we will abbreviate the conventional notation for the V:III chemical ratios from 400:1 to simply read as xe2x80x9c400,xe2x80x9d which means the molar amount of the group V element gas divided by the molar amount of group III element gas).
Since the disappearance of the structural defects thickens the n-type current blocking layer 15 in the vertical direction formed overlying the substrate 1, the amount of leakage current flowing through the current blocking layers 14, 15 is decreased, which in turn increases the laser emission efficiency when a voltage is applied between the electrodes 18, 19.
Further, Mitsubishi Denki Giho (Mitsubishi Electric Advance) Vol. 67, No. 8 (1993), p. 88 points out a decrease of the laser emission efficiency due to a leakage current which does not contribute to the laser emission and which flows along the interface between the mesa structure and the current blocking layer.
The buried semiconductor laser device with the reduced leakage current includes higher laser emission efficiency, good linearities of the higher output characteristic, and an excellent current-voltage characteristic. Accordingly, when the leakage current path width is reduced, the resistance of the current blocking layer increases to provide desirable laser characteristics.
Even when the current blocking layer is formed under the conditions described in the former publication such that the substrate temperature is between 750xc2x0 C. and 800xc2x0 C., and the mixing ratio between the group V element gas and the group III element gas is between 400 and 800, the leakage current path width is quite difficult to be formed in a narrower manner with the excellent reproducibility, and the values of the widths are difficult to be regulated and controlled.
Similarly, in the fabrication of the buried semiconductor laser device formed on the p-type substrate, an n-type InP contact layer is excessively grown to be in contact with an n-type InP contact layer, and a leakage current path width is increased.
As a result, the increased leakage current lowers the laser emission efficiency to worsen the output characteristic and the linearity of the current-voltage characteristic, and the buried semiconductor laser device with the higher output can be hardly fabricated with the excellent reproducibility.
The present invention encompasses buried semiconductor laser devices and methods of manufacturing the same. An exemplary general method according to the present invention comprising forming a mesa structure including a bottom cladding layer, an active layer and a top cladding layer overlying a semiconductor substrate. The mesa structure has at least one side surface extending from the top surface of the mesa toward the bottom cladding layer, with the active layer having an exposed side thereat. The mesa structure also has a skirt surface extending outward from each side surface to cover a portion of the substrate""s surface. The exemplary general method further comprising growing a first current-confinement layer on the mesa""s at least one side surface, with the first current-confinement layer comprising a semiconductor material and having a first conductivity type (e.g., p-type or n-type). A second current-confinement layer is then grown above at least a portion of the first current-confinement layer, the second current-confinement layer comprising a semiconductor material and having a second conductivity type which is opposite to the first conductivity type. The closest spacing distance between the second current-confinement layer and the active layer defines a xe2x80x9cleakage current path widthxe2x80x9d (e.g., Tn or Tp). This spacing distance is normally shown in a cross-sectional plane which is perpendicular to the top surface of the substrate, and which is oriented to provide the smallest width of the mesa. The first confinement layer is grown at a temperature ranging from 610xc2x0 C. to 700xc2x0 C. using a raw material gas comprising a group V element gas and a group III element gas at a molar ratio of the group V element gas with respect to the group III element gas having a value between 50 and 500, inclusive, to provide a value of the leakage current path width ranging from 0.15 xcexcm to 0.60 xcexcm.
As used herein, the term xe2x80x9cgroup V element gasxe2x80x9d is defined as including any precursor gas comprised of molecules, each molecule of the precursor gas comprising one or more atoms of an element listed in the fifth column of the Periodic Table. The term xe2x80x9cgroup III element gasxe2x80x9d is defined as including any precursor gas comprised of molecules, each molecule of the precursor gas comprising one or more atoms of an element listed in the third column of the Periodic Table. A raw material gas may also comprise precursor gases which carry dopant atoms (e.g., elements in the fourth and sixth columns of the Periodic Table). A group V element gas may comprise two or more different precursor gases (each carrying atoms in the fifth column of the Periodic Table), such the combination of a Phosphorous carrying precursor gas and an Arsenic carrying precursor gas. Likewise, a group III element gas may comprise two or more different precursor gases (each carrying atoms in the third column of the Periodic Table), such the combination of an Indium carrying precursor gas and an Gallium carrying precursor gas.
In one exemplary implementation of the present invention (generally described below under xe2x80x9cEmbodiment 1xe2x80x9d), a buried semiconductor laser device is fabricated using the steps of: forming a mesa structure including a bottom cladding layer, an active layer and a top cladding layer overlying an n-type semiconductor substrate; and forming a current confinement structure by growing a p-type current blocking layer (first current confinement layer) and an n-type current blocking layer (second current confinement layer) on each side surface of the mesa structure and preferably on each skirt portion extending from each corresponding side surface. The p-type current blocking layer is fabricated by using a raw material gas comprising a group V element gas and a group III element gas at a molar ratio of the group V element gas with respect to the group III element gas between 50 and 500 inclusive to provide a value of the leakage current path width ranging from 0.15 xcexcm to 0.60 xcexcm.
In another exemplary implementation of the present invention (generally described below under xe2x80x9cEmbodiment 2xe2x80x9d), a buried semiconductor laser device is fabricated using the steps of: forming a mesa structure including a bottom cladding layer, an active layer and a top cladding layer overlying a p-type semiconductor substrate; and forming a current confinement structure by growing a p-type separation layer (first current confinement layer), an n-type current blocking layer (second current confinement layer), and a p-type current blocking layer (third current confinement layer) on each side surface of the mesa structure and on preferably on each skirt portion extending from each corresponding side surface. The p-type separation layer is fabricated by using a raw material gas comprising a group V element gas and a group III element gas at a molar ratio of the group V element gas with respect to the group III element gas between 50 and 500 inclusive, to provide a value of the leakage current path width ranging from 0.15 xcexcm to 0.60 xcexcm.
In accordance with the present invention, the suitable selection of the molar ratio of the group V element gas with respect to the group III element gas suppresses the structural defects such as the hollows and the trenches on the surface of the buried layer, and the buried semiconductor laser device including the current confinement structure having the specified leakage current path width can be fabricated with excellent reproducibility and higher yield.
The buried semiconductor laser device according to the present invention also has larger laser emission efficiency, reduced leakage current, higher optical output, excellent linearity (lack of kinks) of the output characteristic with respect to driving current, and good linearity of the current-voltage characteristic in the lasing region of device operation.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.